The invention relates to a method for determining the charge of an activated carbon container in a tank venting system particularly of a gasoline engine with direct fuel injection wherein the thermal influence of the tank ventilation on the engine and the charge of the activated carbon container which is determined based on the thermal influence wherein an exhaust gas temperature of a catalytic converter is determined with an activated and an inactivated tank venting system and the two temperatures are compared.
Gasoline engines with direct fuel injection include injection valves or injectors which inject the fuel directly into the cylinders of the engine. Depending on the point in time of the fuel injection into the cylinders the operating modes of the engine are designated: If fuel is injected during suction of air into the cylinder, so that the fuel injected has sufficient time to be uniformly distributed throughout the cylinder, the operation of the gasoline engine is called a homogeneous operation. The homogeneous operation is essentially the same as the known combustion methods with fuel injection into the intake ducts. In the ideal case of a homogeneous operation, the fuel is completely burnt. If the fuel is injected only during the compression stroke, that is, shortly before ignition, the fuel does not have sufficient time to be distributed over the whole combustion chamber. Then a mixture cloud is formed near the spark plug while the remainder of the combustion chamber is filled with air. This method of operation is called stratified charge operation. In this case, ideally the whole mixture in the cloud is burnt.
Intermediate states between a homogeneous and stratified operation are also possible if for example fuel is injected into the intake duct or the fuel is injected into the cylinder early during the intake stroke or late during the compression stroke.
In this case, independently of the mixture composition only the mixture in the cloud burns completely. The remaining mixture which is homogeneously distributed throughout the combustion chamber is discharged through the exhaust passage in the form of unburned HC emissions. Through the exhaust duct, the unburnt hydrocarbons reach the catalytic converter in which they are converted, together with the excess air in the stratified charge, to water and carbon dioxide. This conversion in the catalytic converter is exothermic and increases the temperature of the catalytic converter and of the exhaust gas.
When designing a gasoline engine consideration has also to be given to the evaporation of fuel that is hydrocarbons from the tank into the atmosphere. This environmentally adverse effect becomes more prevalent with an increase of the fuel temperature in the tank. With the use of activated carbon containers, which store the hydrocarbons evaporating from the tank, the legal requirements (shed-test) in connection with vaporization losses can be fulfilled. In this case, the tank is vented solely by way of the activated carbon container. However, because of the limited storage capacity of the activated carbon container the activated carbon must be constantly regenerated. When the engine is operating, air is sucked in through the activated carbon container and supplied as a mixture to the engine for combustion. If, for example, 1% of the intake air consists of fuel vapors the mixture composition during homogeneous operation of the engine changes by about 20%. In order to maintain the exhaust emissions within the desired limits and to ensure smooth engine operation the introduction of the fuel vapors into the engine must be properly controlled.
To this end, the engine control unit controls a regeneration valve. The volume flow can be measured almost continuously in the operating range of the valve by means of a performance graph adaptation using the parameters load and engine speed. In certain operating ranges, the regeneration is shut off (idle) or is ineffective (for example, under full load when the vacuum in the intake duct is insufficient, or during stratified charge operation without throttling).
In addition, there is a lambda control arrangement which monitors whether the given emission limits are maintained when during regeneration fuel is added to the intake air. If too much fuel enters the air intake duct, the regeneration flow volume is reduced in order to maintain the operational behavior and the exhaust emission within an optimal range.
The surveillance of the flow volume from the tank venting system is based on the lambda control, which, during homogeneous engine operation, keeps the mixture on the lambda value of 1. The higher the percentage of fuel vapors from the tank venting system in the intake duct, the less fuel should be injected by the injectors in order to keep the engine at a constant operating state.
By way of the adjustment or, respectively, change of the fuel injection volume, the charge of the activated carbon container can therefore be determined. A suitable precondition for such a relationship is an essentially complete combustion of all the hydrocarbons.
During stratified charge operation, the engine must be slightly throttled so that a low pressure is generated and the activated carbon container can be regenerated. The hydrocarbons from the activated carbon container reach the combustion chamber homogeneously distributed in the intake air and are only partially burnt in the engine. The unburned hydrocarbons reach the catalytic converter, are converted therein chemically and increase the temperature of the catalytic converter. Hydrocarbons however cannot be measured by way of a lambda probe since the lambda probe responds only to the oxygen content in the exhaust gas.
The charge of the activated carbon container can therefore not be determined by a lambda probe during stratified charge engine operation.
DE 199 47 080 C1 discloses an apparatus and a method for the regeneration of an activated carbon container which is arranged in the tank venting system of an internal combustion engine. The engine is operated with pressurized air supported direct injection of the gasoline. At the high pressure side of an air pressurizing unit for the injection of the gasoline, a pressure controller is arranged whose discharge air is conducted through the activated carbon container for the regeneration thereof.
DE 196 17 386 C1 discloses a tank venting system for an internal combustion engine with direct fuel injection. In this case, the internal combustion engine includes a pressurized air based injection system wherein, under certain operating conditions of the internal combustion engine, the air for the regeneration of the activated carbon container of the tank venting system is admixed to the atomizing air for the injection system which is generated by means of a compressor.
Finally, DE 197 01 353 C1 discloses a tank venting system for an internal combustion engine wherein the charge level of an activated carbon filter is determined. Depending on the charge level and a predetermined value for a maximum fuel mass flow through the tank venting valve a desired flushing flow is calculated and the actuating ratio for the tank venting valve is adjusted depending on the desired flushing flow. The temperature of the flushing flow and the pressure differential at the tank venting valve are so adjusted, that the lambda deviation of a controller of the lambda control arrangement caused by the flushing does not exceed a predetermined maximum value.
It is the object of the present invention to provide a method by which the charge state of an activated carbon container in the venting system of an internal combustion engine can be determined in a simple way.